Membrane fusion, mediated by viral spike glycoproteins, is a key process in the infection cycle of all enveloped human and animal viruses. The goal of this research program is to understand the structural biology of viral membrane fusion. Although the crystal structures of the ectodomains of many viral fusion proteins have been solved in recent years, the structures of the most critical protein domains that interact with the viral and host cell membranes are not known. These domains were routinely removed from the fusion proteins in order to obtain crystals that are suitable for X-ray diffraction. While the crystal structures of the ectodomains have yielded extremely valuable information about overall domain movements in these proteins, these structures do not explain how fusion proteins mediate the merging of the viral and target membranes. This task is left to the fusion and transmembrane domains. In the forthcoming grant period, we wish to determine the structures of the fusion and transmembrane domains in membranes and we wish to understand the conformational changes and mutual interactions of these domains that lead to membrane fusion. The project will concentrate on the fusion and transmembrane domains of the influenza and human immunodeficiency virus (HIV). The four specific aims are to determine: 1. structure-function relationships of the fusion domain of influenza hemagglutinin. 2. the mechanism(s) that explain(s) the coupling between the insertion of the fusion domain and fusion-promoting perturbations of the lipid bilayer. 3. the structural role of the transmembrane domain in membrane fusion. 4. the membrane structure of the fusion domain of HIV.